Bosch K-Jetronic fuel injection

De-mystifying this simple but very efficent injection system.

Talk of classic fuel systems tends to revolve around brand names like SU, Weber and Stromberg but many of the cars we now regard as bona fide classics these days were built with fuel injection rather than carburettors.

You can thank Bosch for that, the German firm which not only pioneered the development of full engine management and fuel injection but which also made the idea affordable for mass production by simplifying it into the K-Jetronic set-up, which has gone on to become the most popular injection system ever, appearing on cars from most major European makers.

To those steeped in the lore of needles and dashpots, any fuel injection system sounds like an impenetrable mass of electronics, but despite coming after Bosch’s electronically-controlled D-Jetronic system, K-Jetronic was all mechanical. Like most of the best engineering solutions, its strength lay in its elegant simplicity and its basic operation was once described to me as being ‘like a toilet cistern’ which isn’t a million miles away from the truth.

K-Jetronic was born from a desire to create a fuelling system which was more efficient than a simple carburettor and which could deliver finely atomised fuel to each cylinder but which avoided the costs of electronic measuring and control devices.

Central to any injection system is a means of accurately measuring the volume of air entering the engine in order to provide a matched amount of fuel and this is achieved in the K-Jetronic system by means of a movable flap sitting in the intake stream. As the volume of air increases, the flap rises and the system allows more fuel to enter the injectors. The more air, the more fuel and although that’s a very simplified explanation, in a nutshell that’s the basic principle. The ‘K’ stands for the German

Kontinuerlich or continuous since the air is measured constantly and the fuel is metered constantly. The injectors are spraying fuel continuously too, regardless of the position of the valves and pistons in the individual cylinders. The major components of the system include the metering head and fuel distributor as one combined unit, plus the injectors themselves and an extra injector operated from a thermo-time switch to richen the mixture on cold starting. There’s also an electric fuel pump and accumulator, plus a pressure regulator to allow excess fuel to return to the tank, but that’s the extent of the electronics: no ECU, no throttle switch and no electronic injector control.

In operation, the system is pretty reliable and in fact there’s little you can do to maintain it other than ensure the fuel filter is changed regularly and that the fuel doesn’t get contaminated by water. Troubleshooting a K-Jet car is made easier if you know how it works though, so here’s a rundown of the essential components.